The present invention relates to a hydraulic drive system equipped on hydraulic machines such as hydraulic excavators, and more particularly to a hydraulic drive system suitable for load-sensing control in which the discharge rate of a hydraulic pump is controlled to hold a differential pressure between a discharge pressure of the hydraulic pump and a load pressure of an actuator at a setting value.
A load-sensing control hydraulic drive system of the prior art comprises, as described in U.S. Pat. No. 4,617,854, a hydraulic pump of the variable displacement type, at least one hydraulic actuator driven by a hydraulic fluid discharged from the hydraulic pump, a directional control valve for controlling a flow of the hydraulic fluid supplied from the hydraulic pump to the actuator, and discharge control means for controlling a flow rate of the hydraulic fluid discharged from the hydraulic pump. The discharge control means comprises displacement volume varying means for the hydraulic pump, e.g., a drive cylinder unit for driving a swash plate, and a control valve for controlling operation of the drive cylinder unit. The control valve has a pair of drive parts opposite to each other, to which are respectively introduced a discharge pressure of the hydraulic pump and a load pressure of the actuator. The drive part to which is introduced the load pressure is provided with a spring for setting a differential pressure.
When the directional control valve is operated with the hydraulic pump set in a driving state, the hydraulic fluid is supplied for the hydraulic pump to the actuator via the directional control valve for driving the actuator. During this time, the control valve is operated responsive to a differential pressure between the discharge pressure of the hydraulic pump and the load pressure of the actuator for controlling operation of the drive cylinder unit. The discharge rate of the hydraulic pump is thus controlled such that the differential pressure is held at a setting value corresponding to a force of the spring.
In the hydraulic drive system as constructed above, the differential pressure between the discharge pressure of the hydraulic pump and the load pressure of the actuator becomes a unique value in accordance with the spring force. Therefore, the relationship between a stroke of the lever for the directional control valve and an operating speed of the actuator is antinomic such that the maximum speed would be lowered if the relationship is set to give a characteristic of small slope for enlarging the metering region, whereas the metering capability would be deteriorated if the relationship is set to give a characteristic of large slope for increasing the maximum speed. In view of this antinomic nature, the relationship is usually set to give a characteristic of large slope at the expense of the metering region with greater emphasis given to the working capability. Accordingly, even when the control lever of the directional control valve is shifted to a small extent in an attempt to operate the actuator slightly to carry out fine operation of a working member (not shown) driven by the actuator, the fine operation is restricted by the actuator speed dependent on a characteristic of large slope in the conventional hydraulic drive system. Specifically, the characteristic of large slope leads to the increased ratio of the stroke of the control lever to the actuator speed, making it difficult to obtain operation of the working member at a finely adjusted speed. The desired operation of the working member is thus hard to achieve. Further, when carrying out the fine operation, an operator tends to feel much more fatigued with the reduced metering region.
By way of example, such fine operation is needed in the digging work performed by hydraulic excavators. In the exemplified fine operation, a boom, an arm, a bucket, etc. provided as the working members are operated to carefully dig the earth around pipes or the like while keeping the working members from contacting with those pipes buried in the ground.